Our long term goals are to characterize human thymocyte clones at a molecular level in order to further understand the ontogeny of human T cell development. In this regard, we have recently derived clones of immature human thymocytes which are T3+, T4- and T8-. One of these cellular clones, designated CII, does not express mRNA encoding the T4 or T8 molecules nor does it express mature T cell receptor Alpha or Beta mRNA. These cells do express high levels of T Gamma mRNA. Importantly, immunoprecipitation of surface 125I labeled T3 from these cells coprecipitates a novel heterodimeric 44/62 kD structure. As expected, these cells do not express surface Alpha or Beta chains. We have shown that the novel T3 associated 44/62 kD structure could be functional in CII cells because anti-T3 triggering results in proliferation, IL-2 synthesis and cytotoxicity. Our specific aims now are: (1) to further characterize the functional properties of CII clones with respect to recognition of MHC molecules, (2) to isolate and characterize the genes encoding the novel 44/62 kD structure, (3) to prepare monoclonal antibodies to the 44/62 kD structure on a wide variety of normal T cell subsets and leukemic T cells, and (5) to derive additional thymocyte clones at distinct stages of differentiation for analysis of T cell receptor genes.